1. Field of the Invention
The invention is in the field of providing tubular elements generally indicated as ducts, such as e.g. for guiding and protecting cables in the ground. More particularly, it relates to a method and an apparatus for installing a duct around a longitudinal member.
2. Prior Art
A method of the above kind is known from U.S. Pat. No. 5,639,183. The known method is intended for installing a tubular cable-duct around a cable buried in the ground. The method includes steps of freeing a portion of the buried cable from the ground in a forward longitudinal direction of the cable, starting from a free end of the buried cable, and advancing a foremost end and subsequent portions of the cable-duct in the forward longitudinal direction of the cable over the portion of the cable freed from the ground, starting from the free end of the cable. The advancing of the duct is achieved by exerting pulling forces on the free end of the cable by means of a pulling wire passed through the duct via a rearmost end thereof, in combination with the exertion of pushing forces at the rearmost end of the duct. Simultaneously a liquid under pressure may be applied as a kind of drilling liquid, which is fed through the duct from the rearmost to the foremost end of the duct, and which flows out at the foremost end for locally freeing the cable from ground. In this manner, a duct, or a duct consisting of several coupled segments, may be installed underground over a great distance along an existing cable, without much excavation work being required for said purpose. After having been installed, the cable, if so desired, may be pulled out of the duct. An advantage of the known method is that curvy sections are easy to follow, without expensive positioning equipment. Disadvantages, however, are that the duct or duct segment which can be installed in one operation is rather limited in length, and that during operation at first the duct or duct segment has to be laid out above ground in its full length and be provided with a pulling wire of at least the same length. The latter, moreover, makes an installation of a duct directly from a reel, on which it generally will be delivered, impossible. Particularly this aspect makes preparation of the installation process very laborious. Furthermore, a well-sealing T-piece, which is required at the rearmost end of the duct for feeding the liquid under pressure together with passing the pulling wire into the duct, is technically quite troublesome, and in practice will lead to spill of liquid. Finally the known method uses a static pushing force at the rearmost end of the duct(segment). However a considerable part of the pushing force is needed for pushing aside the ground in front of the foremost end of the duct. Since the pushing force available at the foremost end of the duct depends on the loss of force through friction over the length of the duct, the total length of the duct which can be installed with the known method is limited also for this reason.
An object of the invention is to provide a method and an apparatus with which a duct can be installed in the ground, in which the disadvantages of the known techniques discussed above do not occur.
According to a first aspect a method is provided of the kind as indicated above, comprising a step of propelling the duct over a free end and further portions of the longitudinal member. The step of propelling can include, according to this aspect of the invention, a substep of pushing the duct by pushing means which engage locally on and at least partly around an outer surface of the duct in the neighbourhood of, optionally at least at a short distance from the free end of the longitudinal member inside the duct, and a substep of maintaining the free end of the longitudinal member in a substantially fixed position with respect to the pushing means using magnetic means. The magnetic means and the pushing means may be coupled either mechanically or via terra firma, or both. Preferably the magnetic means are electromagnetic means, which preferably include a core member and a solenoid means. A foremost end of the duct is led through the solenoid means. The core member is coupled to the free end of the longitudinal member and then inserted into the foremost end of the duct. Then the duct is pushed further through the solenoid means by the pushing means, and the solenoid means are driven by a current in such a manner that the core member is retained within the attractive range of influence of the solenoid means. In dependency of the kind of magnetic material of the core member the solenoid means may be driven by a direct current or a periodically varying current such as a common alternating current, or any combination thereof, and preferably in a controllable manner. The step of pushing may be carried out by rotating transport means such as caterpillar means. In special cases such as the case in which the longitudinal member is buried in the ground, the method preferably comprises an additional step of feeding a fluid under pressure through the duct, from a rearmost end to the foremost end of the duct, in order to support die propelling step. The duct is preferably supplied from a reel.
According to a second aspect of the invention an apparatus is provided of the kind as indicated above, comprising a duct propelling device which includes pulling means provided with coupling means for a tension-proof coupling to a free end of the longitudinal member, and pushing means for pushing the duct over the free end and further portions of the longitudinal member. The duct propelling device can include, according to the second aspect of the invention, a common housing provided with a duct transit channel. The pulling means include magnetic means provided with a duct transit channel surrounding part and a corresponding core member, preferably a solenoid means and a corresponding core member of a magnetic material, respectively. The pushing means include duct transporting means. The solenoid means and the transporting means have been arranged in the common housing around sections of the duct transit channel. The core member has been provided with coupling means for coupling to the free end of the longitudinal member, and the solenoid means have been arranged for being driven by a current in such a way that during operation the free end of the longitudinal member coupled to the core member is maintainable in a substantially fixed position with respect to the transporting means. In dependency of the kind of magnetic material of the core member the solenoid means have been arranged for being driven by a direct current or a periodically varying current such as a common alternating current, and preferably in a controllable manner. The pushing may be rotating transport means such as caterpillar means. In special cases such as the case in which the longitudinal member is buried in the ground, the apparatus preferably comprises additionally means for feeding a fluid under pressure through the duct, from a rearmost end to the foremost end of the duct, in order to support the operation of the propelling device.
A further object of the invention is to provide a method, with which the propelling step, and preferably also the step of feeding the fluid under pressure are carried out in a more dynamical way, instead of the static way of the known installation technique. In that especially the electromagnetic means are applied advantageously.
According to a third aspect of the invention a method is provided in which the solenoid means are driven by a periodically varying current in such a manner that the core member is alternately attracted and not, at least less strong attracted by the solenoid means. Preferably the core member is of a permanent magnetic material, and the solenoid means are driven by a varying current having a periodically inverting polarity, in such a manner that the core member is periodically attracted and repelled by the solenoid means. In a further preferred embodiment of the method the substep of pushing is carried out by shifting transport means which engage on the outer surface locally around the duct with a relative high friction as to block substantial sliding over the outer surface of the duct in a direction of installation and with relatively low friction for a smooth sliding over the outer surface of the duct in the opposite direction, the shifting transport means being rigidly coupled to the solenoid means. An advantage of such a coupling is that indirectly the pushing means are driven via the electromagnetic means and will need no separate drive means. In a still further preferred embodiment the step of feeding the fluid under pressure is carried out, in which the core member is coupled to the free end of the longitudinal member via spring means and is provided with unidirectionally operating piston means, in such a way that driving the solenoid means with said varying current causes pressure variations in the fluid, which pressure variations propagate through the fluid towards the foremost end of the duct. The furthering effect on the installation of the duct through the generation of such pressure variations in the fluid may still be increased by providing the foremost end of the duct with a spouting element having a shape which is extendable and retractable under such pressure variations.
According to a fourth aspect of the invention an apparatus is provided in which the solenoid means have been arranged for being driven by a periodically varying current for alternately attracting and non-attracting, at least less attracting the core member during installation. Preferably the core member is of a permanent magnetic material and the solenoid means have been arranged for being driven by a varying current having a periodically inverting polarity, for periodically repelling and attracting the core member. In a further preferred embodiment of the apparatus the pushing means are shifting transport means provided with seizing means for seizing locally the outer surface of the duct with a relative relatively high friction as to block substantially sliding over the outer surface of the duct in a direction of installation and with relatively low friction for a smooth sliding over the outer surface of the duct in the opposite direction. The shifting transport means are rigidly coupled to the solenoid means. Preferably, the seizing means include-a mainly hollow cylindrical member provided around a section of the duct transit channel, rigidly coupled to the solenoid means, and provided on the inside with one-way grabber-like elements, in such a manner that a duct present in the transit channel can, at least substantially, freely be moved in the installation direction, but is blocked for movement in the other direction. In a still further preferred embodiment of the apparatus, the core member has been provided with unidirectionally operating piston means for fulfilling a unidirectional valve function for the fluid fed through the duct during installation of the duct and for generating pressure variations in the fluid when during operation the core member is periodically repelled and attracted by the solenoid means. In that the apparatus comprises preferably a spouting member arranged for being mounted on the foremost end of a duct to be installed and for spouting fluid which during operation is fed under pressure through the duct towards the foremost end of the duct, which spouting member has been provided with a shape which is extendable and retractable under pressure variations in the fluid fed under pressure.
The duct can be installed from a reel, without previously pacing out any length of duct;
No need to pass a pulling wire through the full length of duct to be installed;
Feeding of fluid under pressure is easier to realize in a fluid-tight manner;
Exerting the forces on the foremost end of the duct in more dynamic manner achieves better performance (such as realization of larger distances and/or more troublesome stretches of installation);
At least one of the embodiments of the propelling device, i.e., the combination of solenoid means and shifting transport means, allows a very compact, solid and reliable construction, which needs only one (electrical) energy source, and which is very suitable for use in a manhole. From reference [2] a process and an apparatus are known for laying a duct in a furrow in the ground, in which magnetic means are used for, during laying, simultaneously pulling in a cable in the duct.
[1] U.S. Pat. No. 5,639,183;
[2] U.S. Pat. No. 5,464,308.
The references have been incorporated by reference in the present application.